Defects and fluorine diffusion in sodium fluoroberyllate glass: A molecular dynamics study

Abstract

A mechanism for fluorine diffusion in sodium fluoroberyllate fluid and glass is described based on a molecular dynamics study. It is shown that the separation of a F from the neighborhood of a Be occurs only when the Be has a coordination number of five or more. The diffusion mechanism involves a F entering the coordination sphere of a fourfold Be, making it fivefold, after which a different F leaves the neighborhood, making the Be fourfold again. The diffusion mechanism qualitatively explains the concentration dependence of the viscosity in alkali fluoroberyllate fluids. At temperatures below the computer glass transition, where no macroscopic F diffusion occurs during a computer run, some F can still separate from fivefold Be by distances greater than 0.5 Å. (In the glass, about 40% of the Be are fivefold coordinated by F.) These motions have an activation energy of 0.1 eV (compared to 0.7 eV for macroscopic diffusion). It is suggested that such sites give rise to secondary relaxations and anomalous low-temperature properties of glass.